Geological formations contain permeable and impermeable layers. These permeable and impermeable layers are frequently faulted, forming number of geological structures where groundwater is pooled (groundwater basin). This means that 40% of precipitation is soaked by the ground, infiltrates into the permeable layers and pools above the impermeable layers before flowing down into the seas. However, flow of the groundwater is restricted by numerous faults in the formations, and is forced to flow along the fault-lines.
Permeable layers are generally 20-45% in porosity and are therefore capable of retaining a large quantity of water (10-20% of the volume of the formations).
In 1979, a cutoff wall measuring 500 m in length and 16.5 m in height was constructed near the outlet of a groundwater basin in Okinawa and used as an underground dam with the storage capacity of 700,000 tons. Subsequently, a number of underground dams were constructed as listed in the table below.
TABLE 1 ______________________________________ cutoff wall storage capacity length height (water intake) name (m) (m) in tons location ______________________________________ Sunagawa 1,835 49 9,500,000 Okinawa Fukusato 1,720 52 10,500,000 Okinawa Minafuku 500 16.5 700,000 Okinawa Kabashima 59 17 10,000 Nagasaki Usami 129 12.5 1,000/day Fukuoka Unknown 3,850 84 100,000,000 China Unknown 820 24 700,000 Taiwan ______________________________________
These dams are intended to supply drinking water or irrigation water. Amount of intake per day is approximately 250 m.sup.3 which is equivalent to 1/40 of the total storage capacity at Kabashima.
In the conventional nuclear power generation systems employed in Japan, uranium oxide is enriched to approximately 3% either in BWR (boiling water reactor) or PWR (pressurized water reactor) and used as a fuel to generate high temperature water steam, which in turn is used to actuate turbines of the generator. The systems are installed on the surface.
Generally, nuclear power generation requires 1.5 million Kw of energy to obtain an output of 500,000 Kw, and of that total energy, 500,000 Kw is converted into electric energy while the rest takes the form of thermal energy. Primary cooling water is used as a moderator for the neutrons generated from a reactor. The primary cooling water is also circulated in the core to remove the heat generated at the core. Heat-laden primary cooling water is exchanged thermally with the secondary coolant of sea water by means of the condenser and recycled into the core. A power generator with the output of 500,000 Kw requires 180,000 m.sup.3 /h of the sea water as the secondary coolant, which amounts to as much as to 4.32 million m.sup.3 /day. The temperature of the secondary coolant usually rises by about 7.degree. C. and the water is discharged into the sea. Thus, the energy absorbed by the sea water is calculated as follows: EQU Q.sub.1 =432.times.10.sup.10 .times.7 cal/day =1.46 million Kw/sec
In the conventional nuclear power generation system, an extremely large amount of secondary cooling water is discharged into the sea. Further, since the cooling water is not designed to be discharged at a distant offshore, the sea water is warmed up and various abnormal events such as red tide and outbreak of jelly fish have occurred.
Another problem is that as the sea water contains much salt, various parts of the generation system such as the water intake port, condenser and pipes become easily eroded.
Nuclear power plants are generally sited at locations where earthquakes are unlikely to occur. However, since the existing plants are constructed on the surface, essential structures such as reactor are easily subjected to strains due to earthquakes if and when they occur.
Although power plants are preferably sited near urban areas where the demand for power is high, nuclear power plants are usually located in remote areas and are therefore inefficient in terms of energy utilization.
Use of sea water as the secondary coolant is further defective in that it requires an energy E.sub.1 =200.times.9.8.times.10=18,000 Kw to pump up the sea water to the reactor, for example, located at the altitude of 10 m at the rate of 200 m.sup.3 per second. Still another defect lies in that it is difficult to site a nuclear power plant at a location where the tide is high.
In view of defects mentioned above in respect of the prior art, the present invention aims at providing an underground nuclear power generation system which is less susceptible to adverse effects of earthquakes, which does not use the sea water, and which can be sited at a location relatively near the urban areas.